1. Field of the Invention
The present invention relates to a method and apparatus for determining phase correlation between the left (L) channel and right (R) channel signals of a stereophonic audio signal, and more particularly to a method and apparatus for determining an in-phase or opposite-phase correlation present in the channel signals.
2. Description of the Prior Art
Conventionally, in television or radio broadcasting stations or the like, in order to phase the L-channel and R-channel signals of a stereophonic audio signal (hereinafter referred to as R-ch and L-ch signals, respectively) with each other (such that at least signal components of the respective L-ch and R-ch signals which should be localized at the center of a sound field are in phase with each other and have the same amplitude), the phase correlation of the L-ch and R-ch signals is measured. Such a measurement is intended to ensure that stereophonic audio devices which are installed in a broadcasting station have their different sets of terminals (for example, hot (H), cold (C) and ground (G) terminals in a balanced transmission system) connected to each other with a particular polarity-terminal in a set of terminals being connected to a matching polarity-terminal in another set of terminals, as well as to determine the phase correlation, i.e. in-phase or opposite phase associated with a stereophonic audio signal in a produced program.
The measurement of such a phase correlation is often conducted using a stereo sound-image display device called a stereo audio monitor or a stereo monitor scope. FIG. 1 shows one such sound-image display device which comprises a sum and difference circuit and a cathode ray tube (CRT). The sum and difference circuit is connected to receive the L-ch and R-ch signals of a stereophonic signal and generates a sum signal (L+R) and a difference signal (R-L). The CRT is connected to receive the sum and difference signals at Y-axis and X-axis input terminals, respectively such that a Lissajous' figure for the stereophonic signal is displayed on the screen thereof.
In balanced type connectors, there are two different types, i.e. an American type and a European type wherein the assignment of polarities to terminals of a connector can be different between the two types. To determine if there is a matching polarity in the connection between any two of the connectors of audio devices, a sine wave of a single frequency is usually used as a test signal which is applied at one end of an audio system to the left and right channels such that Lissajous' figures at other different points in the system are displayed, thereby enabling the determination of the phase correlation of the L- and R-channels at each of the points. A vertical line (L=R) displayed as shown in FIG. 2a indicates that the L-ch signal and the R-ch signal appearing at a particular point are in phase with each other, and thus shows the possible presence of polarity matched connections in the portion up to that particular point. On the contrary, a horizontal line (L=-R) as shown in FIG. 2b indicates these signals in opposite phase, that is, the presence of erroneous connections. Alternatively, a phase meter may be used in place of the above-mentioned monitor for the determination of such polarity matched connections.
Then, the determination of the presence of an appropriate phase condition or phasing between L-ch and R-ch signals from an audio program is performed by applying the L-ch and R-ch signals, rather than the test signals, to the above-mentioned monitor to produce a sound image therefrom. Usually, an actual stereophonic signal, different from the test signal, has a variety of components to be differently localized in a sound field, and thus may present complex figures as shown in FIG. 2c or 2d on the screen regardless of whether the L-ch and R-ch signals of the stereophonic signal are in phase or in opposite phase.
As will be understood from FIGS. 2c and 2d, there is a higher possibility of erroneous determinations occurring in such a method of determining the phase correlation between L-ch and R-ch signals from a given stereo audio program on the basis of displayed sound images thereof, because when components other than centrally localized components, for example, background music, are prominent in the audio program, the sound image display of the audio program does not result in an image as shown in FIGS. 2a and 2b, thereby rendering the "in phase" or "in opposite phase" determination obscure.
In commercial television or radio broadcasting stations, a CM master tape is made for commercial messages to be aired during a particular day. Since several hundreds of CM tapes produced by CM production companies or the like must be rapidly edited to make a particular CM master tape for a day, the above-mentioned erroneous phase determinations may occur at an even higher rate.
An "in opposite phase" stereophonic CM program, if broadcast, will cause problems in sounds reproduced by receivers. More specifically, defective localization of sounds will occur in stereophonic receivers, while low level unclear sounds will be reproduced in monaural receivers.
In view of the problems mentioned above, an object of the present invention is to provide a method and apparatus which is used to more precisely determine the phase correlation for a given stereophonic audio signal which can include centrally localized components.
Another object of the present invention is to provide a method and apparatus which can be used for determining the phase correlation of stereophonic audio signals from a program such as a CM program.